In the bowl of a food processor, combine the cilantro, mayonnaise, sour cream, garlic, lime juice and salt; blend until smooth.
Salt the steak while you preheat a large cast iron skillet over medium-high heat.
Coarsely grate the potato onto a plate or chopping board.
Add a generous drizzle of oil to the pan and add the grated potato, pressing it into a large, flat pancake on the bottom of the pan.
Cook until deep golden on the bottom, then flip and cook until golden.
Set the rösti aside, add a bit more oil to the pan and cook the steak for 5-6 minutes per side, or until cooked to your liking.
Let the steak rest for 5 minutes, then thinly slice across the grain and serve it on the rösti, topped with sauce.